Ensure that build.rs build-time generation of active selections is idiomatic

[ermo]

Context (#83):

ReillyBrogan
Instead of manually adding an entry for every selection generate a macro at build-time using build.rs and then use that.

I adapted this from what I think was a fairly old example on stackoverflow so I think the Rust code is likely not idiomatic anymore. I can't figure out how to run clippy on build.rs however.